Complementary strand-specific sequences of restriction endonuclease generated fragments of radioactive adenovirus type 2 DNA are used in saturation hybridization-mapping experiments with mRNA obtained from the adenovirus type 2 genome is now in hand which shows the rather complex distribution of early and late gene clusters in the viral DNA. Complementary strand-specific probe DNA is now also being used to characterize the integrated viral DNA sequences in virus transformed cells. This approach affords a more efficient and straightforward determination of the fraction of probe DNA sequences integrated in cellular DNA and the number of copies of those sequences per cell. Efforts are now in progress to develop a practical method of electron microscopy for the high resolution mapping of RNA or DNA sequences on the contour of each complementary strand of the viral DNA. BIBLIOGRAPHIC REFERENCES: Philipson, L., U. Pettersson, U. Lindberg, C. Tibbetts, B. Vennstrom, and T. Persson, 1975. RNA Synthesis and Processing in Adenovirus-Infected Cells. Cold Spring Harbor Symp. Quant. Biol. 39(1) 447-456. Pettersson, U., C. Tibbetts, and L. Philipson, 1976. Hybridization Maps of Early and Late Messenger RNA Sequences on the Adenovirus Type 2 Genome. J. Mol. Biol. (in press).